1. Field of the Invention
The present invention relates to a radiographic device which radiographs a radiological image from radiation transferred through a subject, a radiographic system using the radiographic device, and a control method and a recording medium for the radiographic device.
2. Description of the Related Art
In a medical field, an X-ray radiographing system using radiation, for example, X rays, is known. The X-ray radiographing system includes an X-ray generation device which has an X-ray source generating X rays, and an X-ray radiographing device which receives irradiation of X rays generated by the X-ray source and transferred through a subject and radiographs an X-ray image indicating image information of the subject.
The X-ray radiographing device includes an X-ray image detection device and a console. An X-ray image detection device which uses a flat panel detector (FPD) instead of an X-ray film or an imaging plate (IP) in the related art is put into practical use. The FPD is one in which pixels accumulating signal charges corresponding to a dose of X rays which are incident are arranged in a matrix, and converts the signal charge accumulated in each pixel into a voltage signal using a signal processing circuit so as to detect an X-ray image indicating image information of a subject, and outputs the X-ray image as digital image data. The console performs various settings for the X-ray image detection device, and performs image processes on the image data output by the X-ray image detection device.
As disclosed in JP2008-086358A, in an X-ray radiographing system, in order to suppress an exposure dose of a subject and obtain an X-ray image with appropriate image quality, automatic exposure control (AEC) for controlling exposure of an X-ray image is performed. The AEC is to control an irradiation amount of X rays emitted by an X-ray source such that a cumulative value of doses (cumulative dose) of X rays which are applied from the X-ray source, are transferred through a subject, and arrive at the X-ray image detection device, becomes a target dose (dose target value).
The cumulative dose of X rays is defined by a tube current-time product (mAs value) of an irradiation time (s: seconds) of X rays, and a tube current (mA) regulating a dose of X rays per unit time applied by the X-ray source. As a radiographing condition such as an irradiation time or a tube current, there are general recommended values depending on a radiographed part (a chest, a head, or the like), the sex, the age, and the like of a subject; however, the transfer of X rays is changed depending on individual differences such as a subject's constitution, and thus a dose of X rays arriving at the X-ray image detection device varies depending on subjects even if an irradiation amount from the X-ray source is the same. For this reason, the AEC is performed in order to obtain a dose target value without depending on individual differences such as a subject's constitution. In addition, the AEC is performed, and thereby a subject can be prevented from being exposed to excessive X rays.
In the X-ray radiographing system disclosed in JP2008-086358A, a dose detection sensor which detects a dose of X rays arriving at the X-ray image detection device is provided, and the AEC is performed based on a dose detected by the dose detecting sensor. JP2008-086358A discloses two AEC methods.
The first method is a method in which irradiation of X rays in one radiographing is performed so as to be divided into two irradiations of a pre-irradiation and a main irradiation, a dose of X rays per unit time arriving at the X-ray image detection device is measured by the dose detection sensor in the pre-irradiation, a dose of X rays to be applied in the main irradiation is determined based on the measured value, and X rays are applied at the determined dose. In the main irradiation, since a dose is determined in advance, the X-ray generation device starts irradiation at a tube current and an irradiation time set in advance, measures the irradiation time using a timer, and stops the irradiation at a time point reaching a set irradiation time.
The second method is a method in which a cumulative dose is measured in real time by the dose detection sensor during irradiation of X rays, whether or not the cumulative dose reaches a dose target value is determined, and irradiation from the X-ray source stops at a time point when the cumulative dose reaches the dose target value. In the second method, the X-ray generating device transmits a stop signal for stopping irradiation of X rays at a time point when a cumulative dose of X rays arriving at the X-ray image detection device reaches a dose target value. When the stop signal is received, the X-ray generation device stops the irradiation of X rays.
According to the first method, since, by performing the pre-irradiation, a dose per unit time corresponding to a subject's constitution is measured and a dose in the main irradiation is determined based on the measured value, an appropriate dose corresponding to the subject's constitution can be applied in the main irradiation. According to the second method, since a cumulative dose is measured in real time during the main irradiation, and irradiation of X rays stops at a time point when the cumulative dose reaches a dose target value, an appropriate dose corresponding to the subject's constitution can be applied in the main irradiation. In addition, in a case of the second method, a cumulative dose is measured in real time during the irradiation of X rays, and thus the pre-irradiation may be unnecessary.